With dwindling supplies and increasing costs of energy, its efficient utilization and efficiency of operation of heating systems has become and will remain a major priority. This, in turn, is reflected as a need for highly efficient boilers and highly efficient systems and methods for their utilization. In this regard, it is now known that it is not efficient to utilize a single, large boiler for large areas, such as commercial buildings, in regions of the country where substantial fluctuation in temperature can be expected, such as in the northeast. The reason is that such boilers exhibit a relatively high stand-by loss, which is incurred during the entire time that the boiler is in operation. Moreover, a boiler would typically operate at 10-60% of system load for approximately 85% of the operating time of the equipment. Thus, even a boiler with a stand-by loss which is a relatively low percentage of the maximum load would have a relatively high stand-by loss as a percentage of the actual operating level of the system.
In an effort to reduce stand-by loss, boiler plants have been designed to utilize multiple smaller boilers in place of one large boiler, with additional boilers being switched into operation as the load (i.e. outdoor temperature) and, therefore, the required heat output increases. The stand-by loss for each of the smaller boilers will typically be substantially smaller than that of one large boiler and, since most of the boilers will be turned off a high percentage of the time, the overall stand-by loss is substantially reduced.
Although a large number of small boilers operating in an on/off fashion results in a substantial reduction in standby loss, this is not an entirely efficient mode of operation. High efficiency boilers operate far more efficiently at levels in the range of 50% to 80% of their full output, than they do at full output. Accordingly, on-off operation does not make for more sufficient use of such boilers. It has therefore been suggested that, rather than operating a large number of boilers in on-off fashion, a relatively small number of boilers (e.g. two to four boilers) be operated so that each exhibits a plurality of discrete modulation levels between fully on and fully off. In a typical system of this type, each boiler is turned on at a predetermined firing level of modulation and progresses through a sequence of increasing levels until a predetermined threshold level is reached, at which time the subsequent stage is turned on at its firing level. In order to achieve increased efficiency, a subsequent stage may be modulated up towards its threshold level, before the previous stage is modulated substantially beyond the threshold level, since higher levels of modulation can be expected to be substantially less efficient. When heating demand decreases, the modulation level of the last turned-on boiler is progressively decreased with decreasing demand, until it is turned off. Then, modulation of the previous boiler is progressively reduced, and so forth.
Systems of this type achieve a substantial improvement in efficiently of operation, but they are far from optimum. In an optimum system, each boiler would preferably operate at a continuum of levels between its firing level and full modulation. In addition, efficiency is maximized by operating all of the boilers at their modulation level of highest efficiency for the maximum amount of time. It is therefore preferable to distribute the heating load as equally as possible among all of the boilers. In addition, substantial inefficiency results from the constant on-off cycling of the last boiler, which is turned on, so it is desirable that such cycling be eliminated.
Broadly, it is an object of the present invention to provide a multiple boiler heating system which avoids the shortcomings of known systems. It is specifically contemplated that the inefficiencies of prior systems be avoided and that operation close to optimum be achieved.
It is another object of the present invention to avoid the stand-by loss associated with the use of a single large boiler in a heating system.
It is yet another object of the present invention to avoid the inefficiencies that result from operating multiple boilers in a heating system in an on-off mode of operation.
It is yet another object of the present invention to operate a multiple-boiler system so that all the boilers which are on at the same time operate at their highest level of efficiency.
It is yet another object of the present invention to prevent repeated on-off cycling of the last turned on boiler in a multiple-boiler heating system.
It is also an object of the present invention to provide a multiple-boiler heating system and a method for operating the same which result in a high level of efficiency in utilizing fuel, a high level of reliability, and a relatively low cost.
In accordance with an illustrative embodiment demonstrating objects and features of the present invention, each boiler or stage in a multiple-boiler system is provided with an adjustable firing level of modulation at which the boiler is turned on and an adjustable threshold level of modulation (the MOD point), below which the next stage is disabled from being turned on. A control device for the system continuously compares temperature in the medium being heated to the set point for the system and determines the total change in system output level ("DUTYCHANGE") which would be required to produce a specified temperature within a predetermined time. If DUTYCHANGE is positive, it is distributed as equally as possible among successive stages, which are turned on as needed. Should DUTYCHANGE be negative, it is similarly distributed among the stages which are turned on. However, the reduction in modulation level proceeds backwards from the last stage which was turned on. When the last stage reaches its firing point, it is not turned off until the previous stage drops below its MOD point. The lead stage is turned off as soon as it reaches its firing point.